According to Peter Alexander, in the old days, it probably was a matter of style, since a single trigger pinned high paired with a lock that returns its sear to the same place at rest, half-cock, and full-cock is perfectly adequate. Nowadays, it is often used as a compensation for a less than ideal lock.
Set triggers are often used on target rifles, as they are easily adjusted to a very light trigger pull, via a screw in the triggerplate, while leaving the nose of the sear alone. Adjusting the simple trigger is a gunsmithing job...On the other hand, they are difficult to use with gloves one set and trying to use one unset (if the design allows for it) is a poor substitute for a simple trigger as it has a lot of slack and a hard pull. Generally speaking,the "rule" is that a simple trigger is used for a hunting rifle and a set trigger for a target rifle, BUT the rule is broken as often as not! Plenty of people use set triggers to hunt with, and do just fine.
Basically it boils down to preference and style. Certain makers used set triggers even on smoothbores...